But the particles it sees are lighter than expected.

The evidence for dark matter is comprehensive. We've measured its effect on galaxies and galaxy clusters, and we've seen its imprint in gravitational lenses and the cosmic microwave background. The annoying thing is that we still don't know what it is. All the evidence indicates that dark matter is likely to be a weakly interacting massive particle (or WIMP), but the best we've found when searching for this particle is a few intriguing hints of evidence.

In the latest hint, the people behind a detector that sits deep in a Minnesota mine say they've observed three events that appear likely to be the faint impact of dark matter particles. Unfortunately, by every measure they try, the significance of these events falls short of discovery. We're therefore left waiting for better detectors and more data—just as we were the last time this team announced a similar result.

Since the evidence indicates that dark matter is a particle (after all, it had to be around before there were even atoms in order to influence the cosmic microwave background), researchers have focused on three ways we might detect a particle that doesn't fit into the existing Standard Model. One track involves searches through the data in the LHC, looking for signs that some of the mass produced in a collision is being carried away by something we can't detect. So far, that hunt has come up blank.

Another option is to look for evidence of dark matter particles colliding with each other. There are a lot of these out in the Universe, and when they smack into each other, they should produce some indication of it. But those results are, at best, ambiguous. They certainly don't provide any indication that we're on the verge of definitive evidence for these particles.

So we're left with the third option: direct detection. Although dark matter is usually presented as avoiding any interactions with regular matter, that's not entirely true under every model. In some models, there's a tiny potential for dark matter to interact with the sort of matter we're all familiar with (in technical terms, this is called a cross section). Because it's so small, however, even a massive dark matter particle—remember, classified as a WIMP—that smashes into an atom will only end up giving it a gentle nudge. This makes detecting these interactions very difficult, since so little energy is involved.

Nevertheless, the Cryogenic Dark Matter Search (CDMS) has been trying to detect this faint hint of a nudge for a number of years. To get rid of many sources of background, the detector was buried in a mine in Minnesota. To get the atoms of its detectors (it uses both silicon and germanium detectors) to settle down enough so that the impact of a WIMP would be apparent above the background of normal movement, the detectors were chilled to 40 milliKelvin. And then, with everything in place, the researchers waited, gathering data throughout five years.

In the experiments the team is now announcing, the researchers took data from four runs of the silicon detectors, performed between 2007 and 2008, after they worked a few bugs out of the system in the earlier runs. During that time, most of the events the researchers were able to detect came from electrons bumping around in the detector, but these could be filtered out because they're usually associated with some change in charge. They also detected the decay of some radioactive elements in the mine itself. When all that was eliminated, the remaining sources of background were expected to be stray neutrons produced by cosmic rays, but the detector was unlikely to have seen any of these in the time that it ran (the actual calculations suggest it would see 0.13 such events).

With all that accounted for, there were still three events that stood out from the background. Statistically modeling these results, the researchers found that the results favor a WiMP-and-background explanation for the data over a background-only explanation, but the deviation from background-only hasn't reached the five-sigma standard for discovery. Intriguing, but there is still more work to do.

That explanation isn't the only thing the resarchers can tell about the data. Each of these events deposited on the order of 10 kilo-electron Volts into the detector. Working backward, the team calculates that this implies the WIMP they're detecting has a mass of 8.6GeV. That number is informative for a number of reasons. One is that previous direct detection experiments (primarily DAMA/LIBRA and CoGent) have also found hints of a signal at similar energies. These hints may sound promising, but another experiment based on liquid xenon detectors excluded this mass, and theoretical considerations had suggested that any WIMPS should be heavier.

For the second time this month, we're left with a tantalizing hint of a signal in the search for dark matter. Ultimately, that's something well short of satisfying.

The paper will be published in Physical Review Letters, and it has been made available prior to publication by the CDMS team.

Ars Science Video >

A celebration of Cassini

A celebration of Cassini

A celebration of Cassini

Nearly 20 years ago, the Cassini-Huygens mission was launched and the spacecraft has spent the last 13 years orbiting Saturn. Cassini burned up in Saturn's atmosphere, and left an amazing legacy.

I feel like Charlie Brown trying to kick a field goal. Just when you think you're about to get your foot on the ball Lucy pulls it away. In some ways I'd be happier if they'd been way off the mark. "Nothing to see here, move along." Being so tantalizingly close but apparently not close enough to solidly pull signal from noise is painful.

To get the atoms of its detectors (it uses both silicon and germanium detectors) to settle down enough so that the impact of a WIMP would be apparent above the background of normal movement, the detectors were chilled to 40 milliKelvin. And then, with everything in place, the researchers waited, gathering data throughout five years.

40 mK for 5 years - how do they keep things so cold for so long? Wouldn't the slightest interaction with anything raise the temperature? Wouldn't they face some of the same challenges as maintaining a fusion reaction?

To get the atoms of its detectors (it uses both silicon and germanium detectors) to settle down enough so that the impact of a WIMP would be apparent above the background of normal movement, the detectors were chilled to 40 milliKelvin. And then, with everything in place, the researchers waited, gathering data throughout five years.

40 mK for 5 years - how do they keep things so cold for so long? Wouldn't the slightest interaction with anything raise the temperature? Wouldn't they face some of the same challenges as maintaining a fusion reaction?

Maybe I'm missing something, but once you're down to 40mK, why would it be particularly hard to keep it there? Yeah, you have to make sure you don't have a power outage or breakdown, but is redundancy and UPS so hard to imagine?

Cryocooling should work as well. As the experiment is in a mine shaft there would be few heat sources to bring anything cooled back to temperature, which makes keeping it cool easier.

Ambient temperature in the Soudan Mine is about 10 degrees C. I don't see how that's really helping maintain 4mK. In a more typical lab setting, getting the temperature down from 25C to 10C would be the easy part.

Wow, Dark Matter has come a long way since 2012. That last I read about that in 2012, was that is was just a .... guess, a question from some guy who wondered if there was such a thing as Dark Matter. Now it looks like they have went a long way.

Wow, Dark Matter has come a long way since 2012. That last I read about that in 2012, was that is was just a .... guess, a question from some guy who wondered if there was such a thing as Dark Matter. Now it looks like they have went a long way.

Observational confirmation of dark matter has been around for years. See, for instance, the Bullet Cluster observations from 2006.

IF neutrinos have mass (even just energy mass), wouldn't that also account for dark matter without the need for WIMPs (you would have something a lot WIMPier if it was the neutrinos, pun intended, since the neutrinos probably DON'T have rest mass, but would only have energy mass in this case). Since neutrinos by their nature have energy, and the universe is made up of something like 90% neutrinos, any energy mass would contribute a substantial gravitational effect. Additionally, if they have energy mass, they would in turn be effected by gravitation themselves, meaning that very low energy neutrons could be pulled into orbiting a galaxy, by bending their travel path gravitationally the same way photons are bent from General Relativity. That would account for additional gravitational mass of Galaxies and Clusters.

I suppose the best way to prove that would be to create a spray of neutrinos traveling in one direction, and then seeing if the beam is effected by something large, like the sun, with a slight lensing effect. But how would you create a beam of neutrinos?

IF neutrinos have mass (even just energy mass), wouldn't that also account for dark matter without the need for WIMPs (you would have something a lot WIMPier if it was the neutrinos, pun intended, since the neutrinos probably DON'T have rest mass, but would only have energy mass in this case). Since neutrinos by their nature have energy, and the universe is made up of something like 90% neutrinos, any energy mass would contribute a substantial gravitational effect. Additionally, if they have energy mass, they would in turn be effected by gravitation themselves, meaning that very low energy neutrons could be pulled into orbiting a galaxy, by bending their travel path gravitationally the same way photons are bent from General Relativity. That would account for additional gravitational mass of Galaxies and Clusters.

I suppose the best way to prove that would be to create a spray of neutrinos traveling in one direction, and then seeing if the beam is effected by something large, like the sun, with a slight lensing effect. But how would you create a beam of neutrinos?

But if that was the case... and neutrinos run around every corner of the universe... wouldn't be almost impossible to watch a coherent gravitational lensing?

IF neutrinos have mass (even just energy mass), wouldn't that also account for dark matter without the need for WIMPs (you would have something a lot WIMPier if it was the neutrinos, pun intended, since the neutrinos probably DON'T have rest mass, but would only have energy mass in this case). Since neutrinos by their nature have energy, and the universe is made up of something like 90% neutrinos, any energy mass would contribute a substantial gravitational effect. Additionally, if they have energy mass, they would in turn be effected by gravitation themselves, meaning that very low energy neutrons could be pulled into orbiting a galaxy, by bending their travel path gravitationally the same way photons are bent from General Relativity. That would account for additional gravitational mass of Galaxies and Clusters.

I suppose the best way to prove that would be to create a spray of neutrinos traveling in one direction, and then seeing if the beam is effected by something large, like the sun, with a slight lensing effect. But how would you create a beam of neutrinos?

I started a whole long post, but the TL;DR is that we know enough about neutrinos to know that they can't account for the various observations which make us believe dark matter exists.

Since DM mass is unknown, spurious bumps are more easily confused for the real deal , which is why the usual 5 sigma for so called "look-elsewhere" in particle physics is used. And in particle physics 2-3 sigma results come and go the whole time. IIRC the Fermi-LAT DM galactic core bump was 4 sigma once, but with more data it has started to go away.

But the real problem for these underground data is that astrophysicists managed to set a great deal larger lower bound for any kind of DM in 2011, twice over:

"In a paper to be published on Dec. 1 in Physical Review Letters (available in pdf), Brown University assistant professor Savvas Koushiappas and graduate student Alex Geringer-Sameth report that dark matter must have a mass greater than 40 giga-electron volts in dark-matter collisions involving heavy quarks. [ ...]

The observational measurements are important because they cast doubt on recent results from dark matter collaborations that have reported detecting the elusive particle in underground experiments. Those collaborations – DAMA/LIBRA, CoGeNT and CRESST – say they found dark matter with masses ranging from 7 to 12 GeV, less than the limit determined by the Brown physicists.

"If for the sake of argument a dark matter particle's mass is less than 40 GeV, it means the amount of dark matter in the universe today would be so much that the universe would not be expanding at the accelerated rate we observe," Koushiappas said, referring to the 2011 Nobel prize in physics that was awarded for the discovery that the expansion of the universe is accelerating.

Independently, the Fermi-LAT collaboration arrived at similar results, using a different methodology. The Brown and Fermi-LAT collaboration papers will be published in the same issue of Physical Review Letters."

I can also find theoretical constraints as the article notes. Interestingly, they seem to agree with these two empirical constraints (but then specifically assuming DM is WIMP). In sum:

A less than 40 GeV DM candidate would be an extraordinary claim, and would need extraordinary evidence.

Incidentally, the AMS candidate signal is in immediate trouble too:

"In the future, AMS-02 will precisely measure/separate the cosmic ray electrons and positrons up to ∼ 1 TeV, with which the primary electron spectrum hardening [bump] hypothesis can be unambiguously tested. The spectrum hardening of both primary-electrons and nuclei at ∼ 240 GV, if conﬁrmed by AMS-02 in the future, likely suggestsa common physical origin. The electron spectrum softening at ∼ 1 TeV would thus point to “nearby” but old supernova-remnant-like source(s) with a lifetime <~ 10^13 s, like Geminga and possibly also Loop I."

I.e. the common peak-like behavior of several cosmic ray components, not only electron-positron components but also electrons by themselves and nuclei to boot, makes an unambiguous observation of a DM annihilation peak less likely.

IF neutrinos have mass (even just energy mass), wouldn't that also account for dark matter without the need for WIMPs (you would have something a lot WIMPier if it was the neutrinos, pun intended, since the neutrinos probably DON'T have rest mass, but would only have energy mass in this case). Since neutrinos by their nature have energy, and the universe is made up of something like 90% neutrinos, any energy mass would contribute a substantial gravitational effect. Additionally, if they have energy mass, they would in turn be effected by gravitation themselves, meaning that very low energy neutrons could be pulled into orbiting a galaxy, by bending their travel path gravitationally the same way photons are bent from General Relativity. That would account for additional gravitational mass of Galaxies and Clusters.

First: there aren't enough neutrinos. Second, neutrinos are relativistic particles, so their contribution to the universe's state would be different from normal matter (they are classed as radiation, not matter).

Incidentally, some fun facts that should show how hard the detection process is. The team used lead from ancient Roman ships, because normal lead (that had been exposed to cosmic radiation) is too radioactive. For the same reason, they shipped their detector crystals by ground, because airplanes would expose it to too much radiation.

The expected cross section of WIMPs is so small (around 10^-42m^2), that a particle would have to travel through ~77 light years of lead to have a ~50% chance to hit anything, which is why it's so bloody hard to see them.

Worse, we aren't even sure dark matter is weakly interactive, or composed of a single particle type.

For those of you wondering about cooling, CDMS uses a He3/He4 dilution refrigerator. Then a traditional Helium cryocooler outside of that, and then you are into more pedestrian methods of keeping things cool.

Cooling down is, of course, one of their big challenges in the experiment, but once everything is cool, it is in kind of a steady state. Indeed there are plenty of UPSes to keep things on track. Waste heat from the detector electronics are a bigger problem. The mine environment really doesn't play into the challenges much, though you do need to move waste heat back above ground.

A really cool experiment is when they use nuclear demagnetization [ http://en.wikipedia.org/wiki/Dilution_refrigerator ; last line] to cool just the nuclei - which is all what is needed here. It is too much bother for DM, but imagine the signal-to-noise ratio!

Worse, we aren't even sure dark matter is ...composed of a single particle type.

This is the first thought that struck me. And that thought raised the question, if these aren't dark matter then what are they? Two experiments have detected something that doesn't fit the dark matter model so that would suggest to me they've found something else.

Wow, Dark Matter has come a long way since 2012. That last I read about that in 2012, was that is was just a .... guess, a question from some guy who wondered if there was such a thing as Dark Matter. Now it looks like they have went a long way.

Observational confirmation of dark matter has been around for years. See, for instance, the Bullet Cluster observations from 2006.

It goes back much further than that. The first evidence for DM was discovered in the 1930's and was proposed as a theory by Fritz Zwicky in 1933

"While examining the Coma galaxy cluster in 1933, Zwicky was the first to use the virial theorem to infer the existence of unseen matter, which he referred to as dunkle Materie 'dark matter'. He calculated the gravitational mass of the galaxies within the cluster and obtained a value at least 400 times greater than expected from their luminosity, which means that most of the matter must be dark. The same calculation today shows a smaller factor, based on greater values for the mass of luminous material; but it is still clear that the great majority of matter appears to be dark"

As much as some people would like to think so Dark Matter doesn't seem to be a passing theoretical 'patch' or fad. It consistently arises in every aspect of cosmology again and again and has across the entire history of the field. We certainly don't understand it, and it is always possible our hypotheses about it are off on the wrong track, but SOMETHING pervasive is out there, and it sure as heck behaves like WIMPs.

Yep. It seems like there's some boundary condition in there somewhere that they just can't track down.

I often wonder if perhaps these attempts to extend the models we have now, to explain these heretofore unexplained phenomena, aren't missing something completely different and strange that no one has thought of yet. Perhaps we should get a bunch of young physicists together in a room, dose them all with LSD, and record the results for later detailed analysis to see what the hell they come up with.

But then I remind myself that I'm reading general descriptions of these experiments that have been dumbed down several orders of magnitude (the descriptions, not the experiments) so the interested but unknowledgeable lay person like myself can delude himself into believing he might understand the smallest fraction of what's really going on. So like Barbie I'll just mumble to myself that "math is hard" and go away and stop bothering those of you who actually can understand experiments like this.

40 mK for 5 years - how do they keep things so cold for so long? Wouldn't the slightest interaction with anything raise the temperature? Wouldn't they face some of the same challenges as maintaining a fusion reaction?

We use a dilution refrigerator, and lots of babysitting (I am actually typing this from the bottom the the Soudan mine.)

Quote:

John Timmer, perhaps ADRs would be worth an Ars article about?

ADRs are neat, but they only provide cooling while you reduce the magnetic field. They are not for continuous operation.

@RMGBill

Warm (relativistic) dark matter (like neutrinos) is clearly a component, just a small one. Detailed comparisons of the CMB to the large-scale structure of the universe (both from observations and simulations) indicate that the vast majority of dark matter must have been non-relativistic during the epoch where the universe moved from radiation domination to matter domination and structure began to form. We also know something about neutrinos' interaction cross section as well as (limits on) their mass. As such they would have totally the wrong relic density to be the dark matter we observe.

But if that was the case... and neutrinos run around every corner of the universe... wouldn't be almost impossible to watch a coherent gravitational lensing?

Low energy neutrinos are more numerous than high energy neutrinos. Lower energy neutrinos would more easily have their track warped by gravitational effects around a large mass; if they are warped enough, they'll be pulled into an "orbit" around the mass. From OUTSIDE of the galactic mass + neutrino "orbit", the total mass of the system would still seem to come from the system as a whole; hence, the observable galaxy would appear to have higher mass than could be accounted for by observed non-neutrino matter.

IF neutrinos have mass (even just energy mass), wouldn't that also account for dark matter without the need for WIMPs (you would have something a lot WIMPier if it was the neutrinos, pun intended, since the neutrinos probably DON'T have rest mass, but would only have energy mass in this case). Since neutrinos by their nature have energy, and the universe is made up of something like 90% neutrinos, any energy mass would contribute a substantial gravitational effect. Additionally, if they have energy mass, they would in turn be effected by gravitation themselves, meaning that very low energy neutrons could be pulled into orbiting a galaxy, by bending their travel path gravitationally the same way photons are bent from General Relativity. That would account for additional gravitational mass of Galaxies and Clusters.

First: there aren't enough neutrinos. Second, neutrinos are relativistic particles, so their contribution to the universe's state would be different from normal matter (they are classed as radiation, not matter).

Not enough neutrinos? There are more neutrinos than ordinary matter particles, and it's not close.If particle count were all that mattered, the universe would consist of neutrinos, antineutrinos, and fluff. Some estimates I recall say that fully 98% of all particles in the universe are neutrinos. As for the fact they are relativistic particles....

Relativity states that matter and energy are equivalent; based on the amount of energy a particle has, you could compute a rest mass for that particle. By definition of how neutrinos were first discovered, they took energy out of subatomic events and allowed that energy to radiate, at relativistic speeds. Neutrinos do NOT have mass; they cannot if they are able to move a C. However, they DO have energy. And as such, they SHOULD have mass equivalent via that energy. If a photon (also with no true rest mass) can have it's path warped by a gravitational field, a neutrino should be able to have it's, as well. Both are relativistic particles.

Quote:

Incidentally, some fun facts that should show how hard the detection process is. The team used lead from ancient Roman ships, because normal lead (that had been exposed to cosmic radiation) is too radioactive. For the same reason, they shipped their detector crystals by ground, because airplanes would expose it to too much radiation.

The expected cross section of WIMPs is so small (around 10^-42m^2), that a particle would have to travel through ~77 light years of lead to have a ~50% chance to hit anything, which is why it's so bloody hard to see them.

Worse, we aren't even sure dark matter is weakly interactive, or composed of a single particle type.

Interesting that your figures for WIMPs seem to roughly (within an order of magnitude) match the cross section of neutrinos, as well. And we HAVE detected them.

Warm (relativistic) dark matter (like neutrinos) is clearly a component, just a small one. Detailed comparisons of the CMB to the large-scale structure of the universe (both from observations and simulations) indicate that the vast majority of dark matter must have been non-relativistic during the epoch where the universe moved from radiation domination to matter domination and structure began to form. We also know something about neutrinos' interaction cross section as well as (limits on) their mass. As such they would have totally the wrong relic density to be the dark matter we observe.

Thanks! That actually is a helpful rebuttal for my question (which someone downvoted?? Why would they do that?)

So, what evidence was there that DM had to be non-relativistic? I hadn't seen that (didn't study that back in my Physics undergrad days thirty years ago). I do remember a few years ago Ars had an article that finally started to limit the total mass energy of neutrinos; it showed that there was an upper bound (and the suggestion at the time was that the bound was low enough that Neutrinos probably couldn't account for all Dark Matter, which meant that something else was out there), but I don't recall exactly what the percentages were.

Not enough neutrinos? There are more neutrinos than ordinary matter particles, and it's not close.If particle count were all that mattered, the universe would consist of neutrinos, antineutrinos, and fluff. Some estimates I recall say that fully 98% of all particles in the universe are neutrinos. As for the fact they are relativistic particles....

Not enough for their mass, is perhaps more precisely what I meant to say. Cosmologists work in energy density. The energy density of dark matter is (off the top of my head) about a million times greater than that of the radiation background, at present, while the energy density of the neutrino background is ~.68 that of the radiation background.

Quote:

So, what evidence was there that DM had to be non-relativistic? I hadn't seen that (didn't study that back in my Physics undergrad days thirty years ago).

Relativistic energy has a radically different effect on the expansion of the universe than normal matter. For one thing, relativistic energy density drops according to the fourth power of the expansion scale factor, while matter drops off according to the third power, and since the energy density in turn directly affects the expansion rate, the equations only really work if DM is non-relativistic. Otherwise, dark matter would be an insignificant factor by this point in time: it's density would have dropped too much.

Wow, Dark Matter has come a long way since 2012. That last I read about that in 2012, was that is was just a .... guess, a question from some guy who wondered if there was such a thing as Dark Matter. Now it looks like they have went a long way.

Observational confirmation of dark matter has been around for years. See, for instance, the Bullet Cluster observations from 2006.

Absolutely not.

What *has* been confirmed is that the observations you mention do not fit the 4D calculations for known, observable masses, distances and energies.

Dark matter and dark energy are just fudge factors no less than entropy is a fudge factor for thermodynamic processes (especially irreversible processes). They are there to make our standard 4D calculations balance and "agree" with observations.

They are no different from the fudge factors of 120 years or so ago to make Mercury's orbit calculations agree with observation (a mysterious "something" we couldn't see must be perturbing Mercury's orbit and that factor is XXX). Then Einstein put forth special and general relativity and the equations came within the limits of the observational data -- the mysterious fudge factor was no longer needed.

Relying on unobservables should never be an acceptable thing. However, doing so has become a popular thing in many physics theories of late.

It's rather simple. They're looking in the wrong direction. (No, that's not a sarcastic comment.)

... The team used lead from ancient Roman ships, because normal lead (that had been exposed to cosmic radiation) is too radioactive. For the same reason, they shipped their detector crystals by ground, because airplanes would expose it to too much radiation...

I don't think that's (primarily) because of cosmic radiation; I believe it's because of all the nuclear testing. Everything made since the 40s is slightly radioactive now.

Or does that not apply here?

"Relativity states that matter and energy are equivalent; based on the amount of energy a particle has, you could compute a rest mass for that particle."

That's a tough one to wrap the head around sometimes; for me, someone put it into my head that rest mass roughly equals Newtonion-style mass as in: the equivalent amount of attraction to the Earth's surface. Jus to get a handle on it. While that does mean weight, and is supposed to help you differentiate the two parts of mass; it screwed me up for many years.

Now I think of it as: Everything that exists, has mass, which is split between some percentage of velocity and "weight". I kind of misuse weight here, but that's what's burned into my head.

At one extreme end of the scale you have light, which is still not quite 100% Velocity / 0% Weight. But it is fast and light (no pun intended). It has a little bit of weight, because gravity pulls on it. But not much.

The other end of the scale would maybe be a star, or I guess a whole galaxy. Or something big and slow. Slow, but nothing is perfectly still. Not much movement, but whole lot of weight. Now, if you take that same sized star, and speed it up; it now has more mass than it did before. It's mass, is the sum of it's weight and it's motion.

I'm supposed to be using the term "rest mass" in place of weight, but I think does more to confuse than it does to help those in the process of considering these things. Is it just me?